Piezoelectric crystal holder



Patented July 1, 1941 PIEZOELECTRIC CRYSTAL HOLDER John Boyd Atwood andDe Witt Rugg Goddard,

Riverhead, N. Y., assignors to Radio Corporation of America, acorporation of Delaware Application March 9, 1938, Serial No. 194,790

9 Claims.

This invention relates to an improved form of piezo-electric crystalholder and particularly to methods of supporting the crystal.

An object of this invention is to simplify and improve piezo-electricquartz crystal holders, particularly those used for low frequencycircuits.

A feature of this invention is the means by I which a crystal isretained by a slight depression located at the nodal point therein.

Certain types of quartz crystals used mainly for low frequencies have amode of vibration with a nodal point in the center of the large faces.When these are used for filter purposes, the problem arises as to how tomount them. An air gap holder would be unsuitable, as a change in theair gap would produce a change in the filter characteristics. The bestway to eliminate the air gap is to plate the electrodes on the crystalsurfaces. The crystal must then be supported at the nodal point. In thepast, this support has consisted of a holder with two small pinsopposite each other and making connection with the plating by pressureat the nodal point. This is undesirable, since the crystalcharacteristics are changed if the pins should not be centered correctlyon the crystal nodal point. There is also the disadvantage that thecrystal may shift its location due to jars, perhaps received duringshipment. This would change the crystal characteristics and there is thepossibility of destroying the plating beneath the supporting pins.

Our invention overcomes the above-mentioned difiiculties, as will beseen by referring to the accompanying drawing, in which:

Fig. 1 is a plan view of a piezo-electric crystal which is intended tobe supported at its nodal point;

Fig. 2a is a sectional view of Fig. l, the section being taken on line2a-2a.;

Fig. 2b is a section similar to that of Fig. 2a, except for the mountingmeans;

Fig. 3 is a plan view of a piezo-electric crystal holder;

Fig. 3a is a sectional view of Fig. 3, showing the mounting springs forretaining a crystal shown in Fig. 20.;

Fig. 3b is another embodiment of the mounting springs for retaining acrystal, shown in Fig. 217;

Fig. 4 is a plan View of a circular type of crystal holder;

Fig. 4a is a partial plan view of a holder similar to that of Fig. 4,except that the springs are discshaped;

Fig. 5 is a cross-section of Fig. 4

Fig. 5a is a cross-section of Fig. 4a;

Fig. 6 is a plan View of a still further embodiment of this invention;and

Fig. 7 is a section of Fig. 6; and

Fig. 8 is a section of a crystal holder having an air gap.

Referring now in detail to the figures of the drawing, it will be seenthat the holder is constructed of springs S supported on a block ofinsulating material I. Mounted on springs S is a pair of supporting pinsfor the quartz crystal. These pins may be in the form of small balls D,as shown in Fig. 3a, or a pair of pins E, shown in Fig. 3b, which areprovided with two shoulders of different diameters. When the small ballsD are used, the quartz crystal .is ground out at the nodal point to fitthe small balls, as shown in the depressed portions A of Fig. 2a, or Ain Fig. 1. However, if the small stepped or shouldered pins E are used,a small hole C is drilled through the crystal at the nodal point and isof such a size as to admit only the small diameter of the pin E. Thelarger diameter of the pin then forms a supporting shoulder, the smallerdiameter being employed to prevent the crystal from shifting itslocation. The crystals as shown in Fig. 1 are ground or drilled asdesired, and then to improve their electrical characteristics are platedwith a conducting material, such as for example, gold, to providesuitable electrodes T for the crystal. The halls D or pins E thenprovide connection from the electrodes, as well as a support from whichthe quartz cannot move, due to jars or vibrations. The crystals may bereadily removed and replaced, the construction being such that they mustbe returned to the same place. To insure a good electrical contactbetween the pins and electrodes, it is preferred that pins D and E alsobe gold plated, thus providing good electrical conductivity, as well aspreventing oxidization. As is well known to those skilled in the art,the grinding or drilling of the quartz will change its characteristicsslightly, but once ground, the characteristics of the crystal will notchange.

Referring now to the embodiment shown by Figs. 4 and 5, an insulatingring R has mounted thereon three support springs F, G and H, each ofwhich is equipped with either balls D or pins E, located in the centerof each spring for retaining a crystal Q which is similar in shape tothe crystal shown in Figs. 1 and 2a, or 2b. Although a strip type ofspring is shown, in Fig. 4a and Fig. 5a it is illustrated that any orall of the springs F, G and H could be made in the form of a circulardisc, particularly if spring G is made into a disc and ring R divided inthree parts and bound together by any suitable means. (not shown), itwill provide a shielding effect upon springs F and H, the central springG may be employed as making a common connection between two quartzcrystals Q, as clearly shown in Fig, 5. This type of holder isparticularly useful in certain filter circuits which have a commonconnection for the two crystals.

A still further type of crystal holder is shown by Figs. 6 and '7, whichconsists of three springs J mounted upon an insulating block I witheither balls or pins mounted near the ends of the crystal supports.Three springs are used for the same purpose as in the first type ofholder. This second type of holder has the advantage that the crystalscan be inserted in or moved from the holder somewhat more easily than inthe one shown in Fig. 4. In all types of holders, springs may be made toprovide soldering lugs L to make connections to the plated electrodesthrough the springs. These springs also provide protection against jarsbreaking the quartz as the size of the supporting pins must be small inrelation to the area of the crystal.

Fig. 8 shows an additional embodiment of an air gap type of crystalholder which is particularly adapted to piezo-electric crystaloscillators, in which it is better to provide an air gap in place of theplating provided for use with filter circuits for the reason that whenthe crystals are used as oscillators there is a possibility of theplating wearing away. This, however, could be prevented to some extentby making the plating heavier under the balls D at the support point Ain Fig. 2a or under the larger diameter of the pins E surrounding theaperture C in Fig. 217. For an oscillator, it would be better toeliminate the plating entirely and use an air gap type of holder as inFig. 8. This consists, as before, of the stepped pins E, the smallerdiameter being used to locate the crystal properly at its nodal point.The larger diameter is used as a support and as a spacer to provide anair gap for the electrodes U. The electrodes U are of conductingmaterial, whereas the pins E, in this case, are not necessarilyconducting. The shape of the electrodes U and their mounting methods arenot shown as these will vary depending on the design of the equipmentwith which they are used.

Although only a few modifications of this invention are shown, it is tobe distinctly understood that it should not be limited to the preciseshowing therein,

What is claimed is:

1. A piezo-electric crystal holder comprising a crystal having anaperture located at its nodal point, said aperture extending from onesurface to another and a plurality of spring members, a plurality ofstepped pins secured to one end of said spring members, said pins havingthe stepped portion located on each face of said crystal and the smalldiameter portion of the pins extending through a portion of saidaperture.

2. A piezo-electric crystal comprising a crystal having a concavedepression located at the nodal point on both faces of said crystal, anda plated electrode formed on each face and concave depression of saidcrystal.

3. A piezo-electric crystal holder comprising a piezo-electric crystal,a ring of insulating material surrounding said crystal, a plurality ofmetallic disc-like spring members supported by said ring, each springmember having means for retaining said crystal at its nodal point.

4. A piezo-electric crystal holder comprising at least twopiezo-electric crystals, an insulating ring, a plurality of springmembers located on said ring, at least one of said spring membersforming a common connection for two piezo-electrio crystals, and meanslocated on said springs for retaining said crystals at their nodalpoints.

5. A piezo-electric crystal comprising a crystal having a concavedepression located at the nodal point on both faces of said crystal, andan electrode formed on each face of said crystal and concave depressionby plating.

6. A piezo-electric crystal oscillator holder comprising apiezo-electric crystal having a central aperture, electrodes for saidcrystal, and insulating means located at said aperture to space saidcrystal from said electrodes.

7. A piezo-electric crystal oscillator holder comprising apiezo-electric crystal having a central aperture, electrodes for saidcrystal, and insulating pins located to extend in said aperture to spacesaid crystal from said electrodes.

8. A piezo-electric crystal comprising a crystal having an aperturelocated at its nodal point, means for supporting said crystal at itsnodal point, and a plated electrode formed on each apertured face ofsaid crystal, the thickness of said plated electrodes being increased atthe nodal point of said crystal.

9. A piezo-electric crystal holder comprising two piezo-electriccrystals, a plurality of insulating rings surrounding said crystals, aplurality of spring members located on said rings, at least one of saidspring members being in the form of a disc, said one spring member beinginterposed between said crystals to form a shield and common connectionfor the two piezo-electric crystals, and means located on said springsfor retaining said crystals at their nodal points.

DE WITT RUGG GODDARD. JOHN BOYD ATWOOD.

